Suppose there is a photon with momtum p=h/lambda moving in the positive x-direction. Suppose it collides with an electron at rest and is completely absorbed by the electron, and that after the collision, the electron moves to the right with the same momentum of the photon. This seems necessary because of the conservation of momentum. But how would we anyalzye this in terms of the momentum of the center of mass of the system? Can we even do this since the photon has no rest mass?